One-shot arc lamp with mass of vaporizable wire between electrodes



June 2, 1970 o. D. KERSHAW- ONE-SHOT ARC LAMP WITH MASS 0F VAPORIZA BLE i WIRE BETWEEN ELECTRODES 2 Filed July 15. 1966 1T1VTYTOT DeLmaT' D. KeT's haw His A t'to neg United States Patent Ofiice Patented June 2, 1970 3,515,928 ONE-SHOT ARC LAMP WITH MASS F VAPOR- IZABLE WIRE BETWEEN ELECTRODES Delmar D. Kershaw, Kirtland, Ohio, assignor to General Electric Company, a corporation of New York Filed July 15, 1966, Ser. No. 565,586 Int. Cl. H013 61/12 US. Cl. 313-184 6 Claims ABSTRACT OF THE DISCLOSURE A one-shot short are gap discharge lamp intended to be discarded after a single use. To assure reliable starting at normal voltage and current, a wad of fine refractory wire is provided between the, electrodes and resiliently engages their tips. Upon current flow, the wad or fuzz'ball vaporizes creating a very high temperature in the gas filling which starts the arc.

This invention relates to a high intensity, one-shot, arc discharge lamp, that is, a lamp which is used only once and expended in the process or discarded. The invention is more particularly concerned with a lamp construction and features assuring reliable ignition or starting of the lamp while making minimum demand upon the power supply requirements.

Lamps of the present kind are useful in various applications, for instance in photography, solar simulation testing, and satellite or guided missile optical tracking or control. The useful life of the lamp may vary from a few seconds to a few minutes at most, and the lamp is discarded after the single, use. Since the test in which the lamp is used, or the missile or vehicle in which the lamp is carried, may represent a substantial investment, reliable starting of the lamp is an absolute necessity. At the same time, it is desirable to maintain the weight of the power supply for the lamp as low as possible, the optimum of course. being a lamp requiring no more voltage or power to ignite it than is used to operate it after starting.

The object of the invention is to provide a new and improved one-shot high intensity are discharge lamp having the desirable features previously outlined.

Another object is to provide a lamp construction easy to construct and amenable to mechanized operations in manufacture.

In accordance with the invention, reliable starting of a short are gap high intensity discharge lamp is achieved by providing between the electrodes a wad or mass of fine refractory wire which is maintained in place at least in part by its own resiliency, this mass being sometimes referred to as a fuzzball. When the normal operating voltage of the lamp is applied, the mass of fine wire vaporizes creating a very high temperature in the inert gas filling of the lamp. At the same time the high temperature at the many points of burn-out of the fine wire provide copious electron emission assuring ignition of the are between the electrodes.

The features of the invention believed to be novel are set forth in the claims appended hereto. The invention, however, along with its features and advantages will be better understood from a consideration of the following detailed description to be read in conjunction with the accompanying drawing wherein the single figure illustrates a compact short are gap lamp embodying the invention.

The lamp illustrated in the drawing has a tubular envelope 1 consisting essentially of fused silica, preferably clear, crystal quartz. Tungsten electrodes comprising a larger diameter rod-like anode 2 and a smaller diameter cathode 3 are operatively disposed within the envelope and adapted to sustain a discharge in the short gap between their conical tips. The electrodes are preferably formed of thoriated tungsten but other refractory metal having enhanced emissive properties may also be used. The electrodes are conductively sealed into the lamp envelope by means of thin metal foil portions 4, suitably of molybdenum, about which the quartz at the ends of the tube is collapsed or pinched to form hermetic seals generally known as pinch seals. The foils are welded to the external inleads 5, 6 through which circuit connections are made to the lamp electrodes. The foils are thin enough to deform without rupturing or shaling as the quartz contracts or expands with temperature. The relatively massive generally cylindrical portions or enlargements 8, 9 which are located about midway between the electrode tips and the ends of the arc chamber are proportioned to engage the envelope wall and assure that the arc gap is accurately maintained notwithstanding the high acceleration forces to which the lamp may be subjected in use. The flats 10 in the sides of the enlargements permit gas to circulate readily throughout the lamp. The enlargements 8, 9 also serve as heat sinks and radiators, preventing overheating of the foil seals during operation. The lamp contains a filling of an inert gas or mixture of gases at a pressure for instance. of 1 atmosphere or more. The filling may be introduced by means of an exhaust tube which is subsequently tipped off as indicated at 1'1. A1ternatively, the lamp may be constructed using flushing techniques whereby the need for an exhaust tube. is eliminated.

In accordance with the invention, reliable starting of the lamp is achieved by means of a wad or fuzzball 12 of fine wire placed between the electrodes. The wire is of material which has a higher work function than the emitting surface of the cathode. Since the cathode is thoriated tungsten, tungsten or molybdenum wire is suitable. It is also necessary that the mass of wire retain its resiliency or springy characteristics at the processing temperature of the lamp so that it is firmly retained between the electrode tips. For a lamp processing temperature of about 1100 C., tungsten or molybdenum wire is entirely suitable; platinum, and also zirconium, tantalum, and titanium may also be used from the point of view of resiliency.

It is desirable to have the mass of fine wire or fuzzball burn out completely in operation. Otherwise there may be obstruction of the generated light and possibly instability of the arc. Complete burn-out is achieved through proper choice of wire size and size of wad, that is quantity of wire, for given lamp taking into account electrode spacing and the supply voltage and current available. If any of the electrode spacing, supply voltage, or current is increased, the maximum permissible size of wire also increases. For a given lamp design and supply voltage and current, the upper limit in wire size is set by the need to have the wire burned back completely at the available current. The available current may also set a limit on the number of loops or strands of wire bridging the electrode gap, all of which constitute parallel resistance paths. The lower limit in size is set by the need to have the filament reach vaporization temperature at the available voltage taking into consideration the effective or average length of Wire loop or strand extending between electrodes. In general practical designs of compact lamps such as illustrated in the drawing use wire size in the range from .0003 to .0015 diameter. By way of example, the illustrated lamp has an overall length in the quartz body of approximately 5.5 centimeters, an outer diameter of 6 millimeters, electrode spacing of .030" and operates on a volt, 16 ampere (average) supply. For this lamp, a

fuzzball consisting of 3 feet of .0007-inch diameter tungsten wire was used. The wire was crumpled or rolled into a circular ball of spaced convolutions about 0.2" in diameter, which was then compressed and lodged between the electrode tips. The wad of wire in place between the electrode tips has a waisted cross section.

In accordance with the invention, many small strands of wire, at least or more, are used in preference to a single wire or fuse link. In the example previously described, there are about 50 loops making 100 wire strands or paths extending between electrodes. The use of many small wires increases the reliability of arc ignition because the geometry or spatial distribution of the vaporizing wires result in a high temperature field throughout the discharge region. By contrast, a single wire or fuse link tends to vaporize in explosive fashion and cause excessive turbulence in the inert gas filling which may extinguish the arc in process of formation. With a wad of fine wire or fuzzball, the lamp will ignite at its normal arc current and voltage and, by going to sufficiently fine wire, the lamp may even be started on less than normal current.

An electric circuit connection is made by springy contact between the fuzzball and at least one of the electrode tips. Connection to the other may be made in the same way, or alternatively through a weld. This eliminates any need for joining the electrode assembly end to end and greatly reduces manufacturing costs. By contrast, in oneshot arc lamp constructions using a fuse link, an electrode assembly linked end to end must be sealed into a vitreous envelope and this is a very difficult manufacturing operation, subject to heavy shrinkage or breakage due to unequal contraction of metal and vitreous portions on cooling. The use of a fuzzball avoids this problem. At the same time, reliability is'increased because with so many contact points, the performance of the fuzzball on ignition is not affected even though several of the contact points should fail.

Another advantage of the 'wad of fine Wire or fuzzball is its ability to withstand high gravity or acceleration forces and vibration such as occur upon launch of space vehicles or missiles. Since the fuzzball is elastic and light in weight, itmay deform due to the high G forces but upon removal of the forces, it 'will revert to shape. The fuzzball is springily compressed between the electrodes and contact is maintained during the period of high G forces so that the lamp may be ignited even during such time.

While for most applications a mass of wire rolled into a ball is preferable, in the case of a longer are gap it may be preferable to roll a mass of wire into an elongated cylindrical shape. One end of the wire mass is welded to the anode, and the other end engages the cathode springily to complete the circuit and is restrained in place by engagement with the envelope walls.

When zirconium, tantalum or titanium is used for the 4 fuzzball, a small quantity of oxygen may be added to the filling. The quantity should be less than that required to combine stoichiometrically with the mass of wire. Since these elements form oxides which are highly stable at high temperatures and which do not blacken the bulb, such addition of oxygen may be used to assure rapid and total consumption of the wire mass.

While the invention has been described by reference to a specific preferred lamp construction, the details which have been illustrated and described are intended as exemplary and not in order to limit the invention thereto except insofar as included in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A one-shot self-igniting arc lamp comprising an envelope having a pair of spaced apart electrodes sealed therein and including an ionizable filling, at least the cathode electrode being of refractory metal of enhanced emissivity, said electrodes being adequate to support an arc discharge through said ionizable filling for at least a few seconds, said ionizable filling comprising a substantial proportion of inert gas, and a mass of fine refractory wire of material having a higher work function than the cathode disposed between and engaging said electrodes,

said mass including at least ten strands or wire paths.

2. A lamp as in claim 1 wherein the mass of fine wire is resiliently compressed between the electrode tips and makes springy contact with at least one of them.

3. A lamp as in claim 1 wherein the mass of fine wire is welded to one electrode and makes springy contact with the other.

4. A compact source lamp as in claim 1 wherein the mass of fine wire consists of wire from .0003 to .0015 inch in diameter.

5. A compact source lamp as in claim 1 wherein the electrodes consist of thoriated tungsten and the mass of fine wire consists of tungsten or molybdenum 'wire from .0003 to .0015 inch in diameter.

6. A compact source lamp as in claim 1 wherein the mass of fine wire consists of zirconium, tantalum or titanium, and a quantity of oxygen less than that required to combine stoichiometrically with the mass of wire is included in the filling.

References Cited UNITED STATES PATENTS 9/1961 Albrecht 431- 11/1966 Anderson 43 l--95 US. Cl. X.R. 313-224; 314-34 

